.center {
  margin: 0 auto;
}

.outer {
  width: 300px;
  height: 300px;
  border: 1px solid #ccc;
  background-color: skyblue;
  display: flex;
}

.inner {
  width: 150px;
  height: 150px;
  border: 1px solid #ccc;
  background-color: pink;
  margin: auto;
}